Abstract

The geometry of brittle strain at the Cascadia convergent margin is inferred from small-magnitude earthquakes in the upper crust. This analysis provides quantitative data on part of the nonrecoverable component of active deformation that is reflected in geodetic observations. Our results are consistent with paleomagnetic and geodetic models that invoke northward translation of the Oregon forearc region (Oregon coast block) relative to stable North America. The latitudinal variation in the kinematics of the seismogenic deformation is best explained by Euler poles for motion between the Oregon coast block and the North America plate that are near the eastern Washington-Oregon border. General correspondence between our principal strain directions and those from recent residual global positioning system (GPS) velocities indicates progress in understanding active crustal strain from complimentary perspectives (i.e., seismicity and geodesy). Systematic differences may reflect the patchiness of seismicity relative to the coarse GPS network, the possibility of nonseismogenic permanent deformation, and/or the relative simplicity of the elastic model used to isolate the nonrecoverable deformation from the raw GPS velocities.